fix(position): for SVG elements

Bootstrap tooltips calculate the `width` and `height` of elements
using first getBoundingClientRect and fall back to `offsetWidth` and
`offsetHeight` if it is not available:

https://github.com/twbs/bootstrap/blob/master/js/tooltip.js#L297

This fixes a problem with tooltips/popovers on SVG elements in
Firefox: https://github.com/twbs/bootstrap/issues/5956

This change mimics that behavior.

I have also added a test case for the scenario.

### Buggy behaviour:

![bug](https://f.cloud.github.com/assets/502876/1461166/277e0c68-4499-11e3-84de-ee5bbaa65604.png)

### Fixed behaviour:

![fix](https://f.cloud.github.com/assets/502876/1461167/3228e732-4499-11e3-9a6d-ec3968c0be30.png)